Exploration of the consequences of a high carbohydrate and low protein diet in female broodstock trout

Adequate parental nutrition is essential to maintain high reproductive performances and to produce high quality gametes. Concerning teleost fish belonging to higher trophic level, broodstock have a high protein requirement but no specific requirement for dietary carbohydrates. Nevertheless, in farming practice, the protein-rich fishmeal traditionally used in aquafeed formulation needs to be replaced, and plant-derived carbohydrates appear as a possible substitute. In trout (Oncorhynchus mykiss), two previous studies have found some contradictory results. While a first study had demonstrated that the lowering of the protein-to-carbohydrate ratio in female broodstock ratio had surprisingly improved relative fecundity, hatchability and survival of their progeny at the eye-up stage [1], a more recent study calls into question part of these results as they have not been confirmed [2]. Therefore, it is essential to deepen the exploration of the consequences of a maternal high carbohydrate and low protein nutrition on their reproductive performances, before increasing the fishmeal replacement by plant-derived carbohydrates. To do so, two-year old female trout were fed either a control diet formulated with no carbohydrate and a high protein content (65.7 %) or a diet containing 32.5 % carbohydrate but with a lower protein content (42.9 %) for an entire reproductive cycle. The reproductive performances, the quality of the unfertilized eggs and the development of the progeny (at the embryo and at the yolk-sac fry stages) were carefully monitored. On the one hand, the spawn were of high quality (i.e. more than 90% of unfertilized eggs were viable) and, in average, the egg proximate composition was not affected. The relative fecundity (number of eggs produced per unit of weight) was also increased by the high carbohydrate - low protein nutrition, confirming the first results obtained in trout. On the other hand, the egg fatty acid profile was altered, and in particular, a decrease in EPA (20:5 n-3) content was triggered by the maternal high carbohydrate/low protein nutrition in trout. Because this particular fatty acid is known to play an essential role during fish development, its diminution could be a cause for the lower survival observed at eyed stage. Moreover, females fed the high carbohydrate/low protein nutrition produced spawns whose eggs size were more variable. Altogether, even though the relative fecundity was significantly increased, the high carbohydrate and low protein nutrition in female broodstock could have deleterious consequences on their progeny development. [1] Washburn, B. S., Frye, D. J., Hung, S. S., Doroshov, S. I., & Conte, F. S. (1990). Dietary effects on tissue composition, oogenesis and the reproductive performance of female rainbow trout (Oncorhynchus mykiss). Aquaculture, 90(2), 179-195. [2] Callet, T., Hu, H., Larroquet, L., Surget, A., Liu, J., Plagnes-Juan, E., … & Marandel, L. (2020). Exploring the impact of a low-protein high-carbohydrate diet in mature broodstock of a glucose-intolerant teleost, the rainbow trout. Frontiers in physiology, 11, 303.